Second level of Machine Learning (These topics are likely 30 min each)
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Generalizing from Training Data: We can prove that if we
successfully train on enough randomly choose training data, then
the produced machine generalities well to random examples never
seen before.
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Reinforcement Learning & Markoff Chains: The basic theory needed
to learn to solve some complex multi-step task.
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Dimension Reduction & Maximum Likelihood: How to compress your
data while retaining the key features.
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Convolutional & Recurrent Networks: Used for learning images and
sound that are invariant over location and time.
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Generative Adversarial Networks: Used for understanding and
producing a random data item.
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Back Propagation: We could learn how to use matrix multiplication
to learn the slope to your error space.
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Bayesian Inference: Given the probability of a symptom given a
disease, we can compute the probability of a disease given a
symptom.
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Decision Trees & Clustering: The basics.
Varying Degrees of Parallelizability:
As chips with hundreds to thousand of processors chips dominate the
market in the next decade, it is generally agreed that developing
software to harness their power is going to be a much more difficult
technical challenge than the development of the hardware. Suppose you
must allocated your processors to incoming jobs in the way that
minimizes the average completion time of the jobs. Suppose that some
of these jobs are parallelizable and hence can effectively unitize any
processors that you give it. Others are sequential, so giving it extra
processors is a wa0ste. We prove if you have twice as many processors
as God, you can do almost as well as him simply by spreading your
processors evenly among the jobs that are alive. This result was
considered break through in this area, because no one previously
believed that it could possibly be true. Ten years later it became a
chapter in "The Encyclopedia of Algorithms." We also give a more
complex algorithm that only needs 1+eps times as many processors.
Power Management:
``What matters most to the computer designers at Google is not
speed, but power, low power, because data centers can consume as much
electricity as a city.'' --- Dr. Eric Schmidt, CEO of Google. The
most commonly used power management technique is speed scaling, which
involves changing the speed of each processor. In tandem to
determining at each time, which job to run on each processor, the
operating system will need a speed scaling policy for setting the
speed of each processor.
We consider developing operating system algorithms/policies for
scheduling processes with varying degrees of parallelism in a
multiprocessor setting in a way that both optimizes some schedule
quality of service objective, as well as some power related objective.
Broadcast Scheduling:
We investigate server scheduling policies to minimize average user
perceived latency in systems where multiple clients request data from
a server and these are returned on a broadcast channel. Possibly being
able to satisfy many requests to a common file with a single
broadcast, allows the system to be more scalable to large numbers of
clients. One notable commercial example is movies-on-demand. We
provide new on-line scheduling algorithms and new analysis techniques.
TCP:
The standard for reliable communication on the Internet is the
well-known Transport Control Protocol (TCP). It performs well both in
practice and extensive simulation have been done, but there have been
minimal theoretical results. We evaluate the performance of TCP using
the traditional competitive analysis framework, comparing its
performance with that of an optimal offline algorithm.
Braching Programs:
The most powerful model of computation measuring the
amount of space used by an algorithm is branching programs. It is
represented by a directed acyclic graph of states. At each point in
time, the computation is in a particular state. Imagine that the name
of this state specifies everything the computation knows. For example,
being in state q
Communication Complexity:
Suppose Alice knows some information x and Bob knows y and together
they want to solve some task f(x,y). Communication complexity measures
the number of bits they must communicate in order to achieve this.
Shannon's entropy is a good measure of the amount of information sent.
We prove a number of results in this area. One of them looks at how much a
little advice can help Alice and Bob.
Cake Cutting:
The fair division of resources is an important topic, be it settling a
divorce, resolving international issues such as contested underwater
mining territories, or simply cutting a cake. The task is about
figuring out how to divide the resource so that each of the n
recipient feels they've received a fair portion based on their needs
and desires. The best algorithm achieves this with O(n log n)
operations, where each operation either asks a recipient to either
specify how much they value a particular piece or where they would cut
the cake to produce a piece of a given value. We prove a matching
lower bound. We also reduced the time to only O(n) operations by
allowing the algorithm to flip coins and to provide an approximately
fair solution. Interestingly enough, this work was written about in
the Toronto Star [Feb 12, 2006. pg. D.16]
We give a faster randomized algorithm that is in
https://en.wikipedia.org/wiki/Edmonds-Pruhs_protocol
Greedy and Dynamic Programming Algorithms:
The goal of this research area is to define a model of computation,
prove that it to some extent captures an algorithm paradigm (in this
case greedy algorithms or dynamic programming) because many of the key
algorithms in this paradigm can be implemented in the model, and then
to prove that other computational problems cant be solved efficiently
in this model. We have a number of results in this area.
Embedding Distortion:
The adversary places n points in the plane. For every pair of points,
he tells me the distance between each pair distorted by factor of at
most 1+epsilon. My task, without knowing the original placement, is to
again place the points in plane in a way that respects the stated
distances within a factor of 1+epsilon'. Surely for small epsilon and
reasonably large epsilon', the problem is easy. Surprisingly, we did
prove that the problem is NP-hard when 1+epsilon is 3.62 and
1+epsilon' is 3.99. My conjecture, is that it is poly-time for
epsilon' > 2 epsilon and NP-hard for epsilon' < 2 epsilon.
Time-Space Trade-off Lower Bounds for st-Connectivity on JAG
Models:
The computational problem st-connectivity is to determine whether
there is a path from vertex s to vertex t in a graph. The model
considered is the JAG (``jumping automaton for graphs'') introduced by
Cook and Rackoff in which pebbles are moved around the vertices of the
input graph. It is a very natural structured model and is powerful
enough so that most known algorithms can be implemented on it. We
prove tight lower bounds on how the required time increases
exponentially as the space (# of memory cells) available decreases.
30 Anniversary of my PhD. Please humor me.
Complexity Classes:
Instead of studying classes of decision problems based on their
computation times, Papadimitriou, Schafer, and Yannakakis categorized
search problems into a number of complexity classes. This is
particularly important when the associated decision problem is not
known to be in P nor to be NP-complete and when the object being
searched for is known to always exist. Our paper studies these
classes and proves some equivalences and separations among them.
Circuit Depth:
Parallel computation is important but not well understood. The next
significant hurdle is proving that there is a problem that cannot be
solved on a circuit (and, or, not) with only O(log n) depth
(i.e. separate NC^1 from NC^2). Towards this goal, Karchmer, Raz, and
Wigderson suggested the intermediate step of proving a lower bound on
a slightly simplified version of their communication game
characterization of circuit depth, which they call the Universal
Composition Relation. Our paper gives an almost optimal lower bound
for this intermediate problem.
Comparisons Between Models of Parallel Computation:
For my masters, I used Ramsey theory and information theory to
separate to models of parallel computation. This provided a greater
understanding of the partial information a processor learns about the
input.
Job DAG:
We consider the problem of computing bounds on the variance and
expectation of the longest path length in a DAG from knowledge of
variance and expectation of edge lengths. We present analytic bounds
for various simple DAG structures, and present a new algorithm to
compute bounds for more general DAG structures. Our algorithm is
motivated by an analogy with balance of forces in a network of
``strange'' springs. The problem has applications in reliability
analysis and probabilistic verification of interface systems.
* Warning:
I do not do TCP AT ALL.
I am a theory person. I prove theorems.
Lower Bounds
For both practical and theoretical reasons, we would like to know the
minimum amount of time (or space) needed to solve a given
computational problem on an input of size n. An upper bound provides
an algorithm that achieves some time bound. A lower bound proves that
no algorithm correctly solves the problem faster no matter how clever.
Doing this on general models of computation (eg. in JAVA or a Turing
Machine) is beyond our reach. For this reason, researchers often prove
lower bounds on weaker modes of computation.
Tight Lower Bounds for st-Connectivity on the
Node Named Jumping Automata for Graphs NNJAG Model
It is a structured model,
- It moves pebbles around the input graph, ie. It either remembers the entire name of a node or nothing about it.
- it cant go to a node without walking there first. (The jumping is to nodes already containing pebbles.)
It can do arbitrary calculations on info learned and remembered, but it has limited memory space.
With O(nlogn) memory it could traverse and memorize the entire graph in linear time and then solve the halting problem.
With less space, it cant remember which nodes it has been to before and hence wanders aimlessly.
No more work has been done in this area because:
- In this model, the results for directed graph are tight and good for undirected (Me)
- In any stronger model, lower bounds are too hard.
https://www.eecs.yorku.ca/~jeff/research/jag/ck_journal.pdf
https://www.eecs.yorku.ca/~jeff/research/jag/talk.pdf
Randomness
Handling Data
(These were two side papers. I
don't supervise people in data bases)